Use Task.set_status to mark tasks as dismissed

Calling Task.toggle_dismiss caused the task to reactivate if it was
already dismissed. (E.g., when selecting and dismissing deeply
nested task structures.)

GTG/gtk/browser/main_window.py

@@ -1393,7 +1393,7 @@ def on_mark_as_done(self, widget=None):
 
     def on_dismiss_task(self, widget=None):
         for task in self.get_pane().get_selection():
-            task.toggle_dismiss()
+            task.set_status(Status.DISMISSED)
 
     def on_reopen_task(self, widget=None):
         for task in self.get_pane().get_selection():